1. Field of the Invention
The present invention relates generally to electronic systems and more specifically to systems and methods related to input voltage sensing fir a switching power converter and a triac-based dimmer.
2. Description of the Related Art
Electronic lighting devices, such as light-emitting-diodes (LEDs) and compact fluorescent lamps (CFLs) provide more energy efficient light relative to incandescent light bulbs. Accordingly, the more efficient electronic lighting devices are increasingly replacing incandescent light bulbs in lighting systems.
Conventional dimmers used to dim incandescent light bulbs often include a triode for alternating current (“triac”). A triac-based dimmer sets a dimming level by phase cutting an alternating current (AC) input voltage at a particular phase angle to decrease the average amount of power delivered to a load, such as lamp. Incandescent bulbs naturally respond to the phase cut input voltage by reducing their light output. Many electronic lighting devices, such as LEDs and CFLs, are constant current devices, meaning that the electronic lighting devices have a particular brightness for a particular current value. Thus, lighting systems that include electronic lighting devices often utilize a controller to convert the input voltage into a voltage and current useable by the electronic lighting devices.
To respond to the dimming level set by the phase angle of phase cut input voltage, the controller ideally correctly detects the phase angle of the phase cut voltage and regulates a voltage and current for the electronic light devices commensurate with the dimming level. However, determining the phase cut angle of the input voltage has proved challenging, especially when attempting to retain the energy efficiency of the electronic lighting devices.
Ideally, a triac-based dimmer connects and disconnects the input voltage to a load one-time each during a single half-cycle of the input voltage. However, without intervention a triac-based dimmer tends to improperly connect and disconnect the input voltage to the electronic lighting device multiple times during a single half cycle of the input voltage. This oscillation occurs because once a triac begins conducting, the triac will continue to conduct until current flowing through the triac falls below a hold current value. Electronic lighting devices often draw only 10% of the current drawn by an incandescent light bulb while producing an equivalent amount of light. Consequently, without intervention the electronic lighting device may draw less than the hold current causing the triac to prematurely connect and disconnect the input voltage to the load during a phase cut when the triac should remain nonconductive. Additionally, some circuits can cause the triac to prematurely disconnect after the phase cut of the input voltage.
Improper functioning of the triac-based dimmer can cause difficulties in detecting the zero crossings of the input voltage. Improper or non-detection of the zero crossings of the input voltage can be disruptive to the proper operation of the controller in controlling the electronic lighting devices.